How to regenerate private key using EdDSA/Ed448 signature algorithm?

Question

Here is how I generated the key pair:

var keyPairGenerator = KeyPairGenerator.getInstance("Ed448");
keyPairGenerator.initialize(448, SecureRandom.getInstanceStrong());
var keyPair = keyPairGenerator.generateKeyPair();
var privateKey = keyPair.getPrivate();
var publicKey = keyPair.getPublic();
byte[] privateKeyBytes = privateKey.getEncoded();
byte[] publicKeyBytes = publicKey.getEncoded();

However, when I tried to regenerate the keys again, I got an exception:

var keyFactory = KeyFactory.getInstance("EdDSA");
var edECPoint = byteArrayToEdPoint(publicKeyBytes);
var edECPublicKeySpec = new EdECPublicKeySpec(new NamedParameterSpec("Ed448"), edECPoint);
var publicKey = keyFactory.generatePublic(edECPublicKeySpec);
var edECPrivateKeySpec = new EdECPrivateKeySpec(new NamedParameterSpec("Ed448"), privateKeyBytes);
var privateKey = keyFactory.generatePrivate(edECPrivateKeySpec); // generatePrivate() throws exception

The exception stacktrace:

Exception in thread "main" java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: key length is 73, key length must be 57
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAKeyFactory.engineGeneratePrivate(EdDSAKeyFactory.java:129)
    at java.base/java.security.KeyFactory.generatePrivate(KeyFactory.java:390)
    at io.fouad.Main.main(Main.java:48)
Caused by: java.security.InvalidKeyException: key length is 73, key length must be 57
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAPrivateKeyImpl.checkLength(EdDSAPrivateKeyImpl.java:84)
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAPrivateKeyImpl.<init>(EdDSAPrivateKeyImpl.java:61)
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAKeyFactory.generatePrivateImpl(EdDSAKeyFactory.java:171)
    at jdk.crypto.ec/sun.security.ec.ed.EdDSAKeyFactory.engineGeneratePrivate(EdDSAKeyFactory.java:127)
    ... 2 more

and here is some utilities I copied from OpenJDK repo:

private static EdECPoint byteArrayToEdPoint(byte[] arr)
{
    byte msb = arr[arr.length - 1];
    boolean xOdd = (msb & 0x80) != 0;
    arr[arr.length - 1] &= (byte) 0x7F;
    reverse(arr);
    BigInteger y = new BigInteger(1, arr);
    return new EdECPoint(xOdd, y);
}

private static void reverse(byte [] arr)
{
    int i = 0;
    int j = arr.length - 1;
    
    while(i < j)
    {
        swap(arr, i, j);
        i++;
        j--;
    }
}

private static void swap(byte[] arr, int i, int j)
{
    byte tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
}

Answer 1

Since privateKey.getEncoded(the instance will be of EdDSAPrivateKeyImpl type) will return a key in PKCS8 format, you should use PKCS8EncodedKeySpec :

var keyFactory = KeyFactory.getInstance("EdDSA");
var pkcs8EncodedKeySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
var privateKey = keyFactory.generatePrivate(pkcs8EncodedKeySpec);

Otherwise you would have to unwrap the the sequence yourself and get PrivateKey (this is OctetString) as described in RFC5958-section2. Then you could use EdECPrivateKeySpec.

Answer 2

Method #1:

var keyPairGenerator = KeyPairGenerator.getInstance("Ed25519");
var keyPair = keyPairGenerator.generateKeyPair();
var privateKey = (EdECPrivateKey) keyPair.getPrivate();
var publicKey = (EdECPublicKey) keyPair.getPublic();
byte[] privateKeyBytes = privateKey.getBytes().orElseThrow();
var edecPoint = publicKey.getPoint();
BigInteger y = edecPoint.getY();
boolean xOdd = edecPoint.isXOdd();

// reconstruct the keys using (privateKeyBytes), (y) and (xOdd)

var keyFactory = KeyFactory.getInstance("Ed25519");
var edECPublicKeySpec = new EdECPublicKeySpec(NamedParameterSpec.ED25519, new EdECPoint(xOdd, y));
var publicKey2 = keyFactory.generatePublic(edECPublicKeySpec);
var edECPrivateKeySpec = new EdECPrivateKeySpec(NamedParameterSpec.ED25519, privateKeyBytes);
var privateKey2 = keyFactory.generatePrivate(edECPrivateKeySpec);

---

Method #2:

var keyPairGenerator = KeyPairGenerator.getInstance("Ed25519");
var keyPair = keyPairGenerator.generateKeyPair();
var privateKey = keyPair.getPrivate();
var publicKey = keyPair.getPublic();
byte[] privateKeyBytes = privateKey.getEncoded();
byte[] publicKeyBytes = publicKey.getEncoded();

// reconstruct the keys using (privateKeyBytes) and (publicKeyBytes)

var keyFactory = KeyFactory.getInstance("Ed25519");
var publicKey2 = keyFactory.generatePublic(new X509EncodedKeySpec(publicKeyBytes));
var privateKey2 = keyFactory.generatePrivate(new PKCS8EncodedKeySpec(privateKeyBytes));